The present invention relates generally to an electrical connector and, more particularly, to a connector for a coaxial cable which has precise matched impedance characteristics over the cable termination section of the connector.
It is known in the art of coaxial cable terminations to utilize a metal sleeve around the cable dielectric which functions as a support during insertion of the cable into the coaxial contact assembly of the termination. Typically in such arrangements the metal sleeve does not provide a predictable electrical path through the termination so that the matched impedance characteristics of the termination are compromised, which can result in RF signal losses occurring at higher frequencies, on the order of one GHZ and above.
U.S. Pat. Nos. 3,701,086 and 4,340,269 disclose coaxial connectors having metal sleeves which are fit between the inner dielectric core of the coaxial cable and the outer conductor or braid of the cable. An outer sleeve is then crimped down around the outer conductor and inner sleeve to provide a secure electrical connection between the cable outer conductor and the conductive shell of the connector. In these arrangements, the inner sleeve over which the outer conductor of the cable is laid is relatively thin, and is crimped when the outer sleeve is crimped onto the outer conductor, resulting in both the outer conductor of the cable and the inner sleeve being deformed. As a consequence, the outer conductor of the cable and inner sleeve no longer have the desired circular configuration required for exact matched impedance characteristics, which depend in part upon the inner diameter of the outer conductor and the outer diameter of the center conductor to be carefully maintained in distance, circular configuration and concentricity. Further, the points of positive electrical contact among the sleeve, cable braid, and coaxial contact assembly may not be in the required locations for optimum high frequency performance. If the coaxial cable utilizes a very soft dielectric core, the problem of maintaining a matched impedance over the termination region of the cable with the coaxial connector assembly is even more difficult because the outer conductor of the cable will deform more readily when the outer sleeve is crimped down over the outer conductor of the cable onto the inner sleeve.
U.S. Pat. No. 3,196,382 discloses a coaxial cable connector in which the sleeve inserted between the dielectric core and the outer conductor of the cable is a rigid bushing which is not deformed during the crimping operation so that the shape of the sleeve is not altered. However, during the crimping operation it is possible that bushing may shift axially so that the forward end of the sleeve may become spaced behind a step in the interior of the connector shell. To obtain matched impedance at high frequencies it is critical that there be a good electrical contact between the forward end of the bushing and the shell, and that the front of the bushing be precisely aligned or flush with said step in the shell. A RF signal travels along the surface of a conductor. In the case of a series of conductors, this means that the signal travels along the conductive surface from contact point to contact point rather than through the conductors as with conventional power or low frequency signal. If the contact points are not arranged to create an optimum surface path in the outer conductor circuit, usually the minimum possible surface length through the section, excessive inductance and/or resistance are introduced in the outer circuit causing RF signal loss. The electric field emanating from the surface of the center conductor circuit also "sees" the total length of the outer conductor from contact point to contact point and is affected by it. Although additional counterbores or steps in the outer circuit may not add length, these features introduce sections of unmatched impedance and capacitance which also cause RF signal loss. Sensitivity of the RF signal to all of these conditions increases with frequency; therefore, the more of these conditions present and the higher the frequency, the greater the RF signal loss. Because the bushing in the coaxial connector disclosed in U.S. Pat. No. 3,196,382 is capable of moving axially, the connector may not have the degree of matched impedance required for high frequency applications.
It is the object of the present invention to provide an improved coaxial connector termination which has precise matched impedance characteristics over the termination region of the assembly.